The subharmonic instability of a two‐dimensional compressible boundary layer over an insulated flat plate is analyzed using the Floquet model. The resulting problem is solved numerically by using both finite differences and the computer codesuport. Results are presented for subsonic, transonic, and supersonic flows. For supersonic flows results for the first and second modes are presented. The effects of Mach number, spanwise wave number, amplitude of the primary wave, Reynolds number, and frequency are studied.